It is widely accepted that patients on multiple medications are susceptible to pharmacokinetic drug-drug interactions when one of the drugs interferes with the activity of key enzymes involved in drug metabolism, chiefly enzymes in the cytochrome P450 family. In the past decade the FDA has commonly required companies to conduct clinical trials to determine the result of such drug-drug interactions. Some drugs such as ketoconazole have been shown to inhibit the activity of individual P450 enzymes; while other drugs such as rifampicin or omeprazole induce expression of higher P450 levels in the liver. These interactions can require dose adjustments to avoid having elevated drug concentrations and potentially increasing toxicities or having insufficient drug levels to be efficacious. Existing in vitro tools can reasonably predict compounds that are likely to cause most of the common metabolism-based drug-drug interactions. However, tools are not currently available to distinguish between the major CYP3A enzymes, CYP3A4 and CYP3A5. These two enzymes are implicated in the metabolism of more pharmaceuticals than any other P450s. Alteration of CYP3A4/5 activity can have serious consequences and drugs that are metabolized through this pathway often come with warnings: not to take with grapefruit juice, ketoconazole, St. Johns Wart, and numerous other CYP3A inhibitors or inducers. CYP3A5 may be the most important genetic contributor to interindividual and interracial differences in the metabolism of prescription drugs. Individuals with at least one gene for the active CYP3A5*1 allele express large amounts of functional CYP3A5, whereas CYP3A5*3*3 causes a splice variant that results in minimal CYP3A5 expression. Approximately 60% of African Americans and 30% of Caucasians have at least one *1 allele and express roughly equivalent levels of CYP3A5 and CYP3A4. CYP3A4 and CYP3A5 activity cannot be clearly differentiated in biologically relevant samples. Currently, the data for CYP3A inhibition of almost every approved compound is generated using a CYP3A assay measuring testosterone hydroxylation or midazolam hydroxylation by pooled human liver microsomes. While both of these substrates are metabolized by CYP3A4 and CYP3A5, the resulting inhibition constants primarily reflect CYP3A4 because multiple donor pools of hepatic microsomes contain approximately five to six times more CYP3A4 than CYP3A5. We have identified the first highly selective substrate of CYP3A5 and propose to build a selective CYP3A5 assay capable of quantifying CYP3A5 activity and inhibition in hepatic microsomal incubations.